Displays can be created from an array of organic light emitting devices (“OLEDs”) each controlled by individual circuits (i.e., pixel circuits) having transistors for selectively controlling the circuits to be programmed with display information and to emit light according to the display information. Thin film transistors (“TFTs”) fabricated on a substrate can be incorporated into such displays.
Mobility characterizes the responsiveness of a charge carrier in the presence of an electric field. Mobility is generally expressed in units of cm2/V s. For transistors, the mobility of the channel region provides a measure of the performance of the transistor “on” current, e.g., the current that can be supplied by the transistor. In thin film transistors, a layer of semiconductor material is generally utilized to form the channel region.
Development of OLED display devices is challenged by the demand for a suitable drive transistor in the pixel circuits. Amorphous silicon (a-Si), the transistor channel material that sources the voltage to switch AM-LCD pixels, has a low mobility (˜0.1 cm2 V−1 s−1). Organic semiconductor channel materials are attractive for use as pixel circuit drive transistors for their homogeneity, low cost, and the variety of means by which they can be deposited, but their best mobilities are similar to that of a-Si. In a typical TFT architecture, low-mobility channel layers would require a large source-drain voltage to drive the necessary current. This consumes power in the transistor (as opposed to light production in the OLED), compromising the power savings.
P-type a-Si TFTs can have even lower mobility values, and can be as low as 0.01 cm2 V−1 s−1.